(a) Field of the Invention
The present invention relates to a method and an apparatus for initial access in a mobile communication system. More particularly, the present invention relates to a method and an apparatus for initial access in a millimeter-wave-based mobile communication system.
(b) Description of the Related Art
Currently, a fourth-generation (4G) mobile communication system may support a data transmission capacity up to 1 Gbps using a carrier aggregation technology in a plurality of frequency bands of less than 6 GHz. In the carrier aggregation technology, the plurality of frequency bands may be divided into a plurality of carrier components (CCs), and data may be simultaneously transmitted using the plurality of CCs, such that data transmission capability may be increased.
A terminal performs initial access using one of the plurality of CCs to set up connection with a base station. This CC becomes a primary carrier component (PCC). The terminal may have one PCC and a plurality of secondary carrier components (SCCs). The terminal uses the PCC to transmit and receive control messages and transmit control signals such as a scheduling request to an uplink, hybrid automatic repeat request (HARQ) feedback, channel quality indicator (CQI) feedback, and the like. In the case of intending to change the PCC, a handover procedure is used.
A millimeter wave (of 6 GHz or more) based mobile communication system uses a wider frequency band in a high frequency band. In addition, the millimeter-wave-based mobile communication system uses a beamforming technology due to frequency characteristics such as a very strong straight propagation feature.
In consideration of the straight propagation feature of the millimeter wave frequency, one cell consists of a plurality of beams, each of which supports a service using an entire frequency bandwidth of the base station. In this environment, since a frequency bandwidth of each beam is wide, the frequency bandwidth of each beam is divided into frequency allocations (FA) of a plurality of small frequency bandwidths, and a service is performed using each FA as a sub-beam (SB). That is, the base station consists of a plurality of beams, each of which is divided into a plurality of SBs.
In the millimeter-wave-based mobile communication system as described above, the terminal selects a beam having the best signal in initial access process for setting up connection with the base station, and selects any of the plurality of SBs in the selected beam to perform initial access to the base station. The SB performing the initial access to have connection with the base station may become a primary sub-beam, and the base station may allocate a plurality of secondary sub-beams to the terminal after the initial access procedure is successfully performed. Therefore, it is required for the base station to dispose the terminal well between the plurality of SBs to increase efficient utilization of resources.
In the existing carrier aggregation technology, in the case of intending to change the PCC, a handover procedure is used. Since this is performed after the terminal arbitrarily selects the primary sub-beam (PSB) to perform the initial access procedure and is allocated with a dedicated resource of the PSB, complexity of a signal and a service delay may be generated. Particularly, since the millimeter-wave-based mobile communication system divides the wide bandwidth into the plurality of SBs and the terminal selects any of the plurality of SBs to perform the initial access procedure, a problem that accesses of the terminal are concentrated on a specific SB may occur.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.